Cathode ray sweep circuits



Nov. 21, 1939.

SOURCE 0F SYN. SIGNALS FLA TE CURRENT F. R. NORTON 2,189,364

CATHODE RAY SWEEP CIRCUITS Filed Feb. 25. less PLA TE VOL T4 65 TIME /N5 N TOR ERNORTON ATTORNEY Patented Nov. 21, 1939 S "EES Bell TelephoneLaboratories, Incorporated,

New York, N. Y., a corporation of New York Application February 25,1936, Serial No. 65,607

13 Claims.

This application relates to cathode ray devices cuits suitable fortelevision scanning and for genand more specifically to cathode raysweep cireral oscillographic purposes.

In order to produce proper linear scanning in cathode ray tubes, it isnecesary to have sweep circuits capable of generating saw-tooth waveforms. To produce wave forms of this general shape, circuits in theprior art have been devised which charge up a condenser through aresistance or through a saturated diode, the condenser being dischargedat suitable intervals by a suitable gasfilled electron discharge devicesuch as a Thyratron tube actuated by received synchronizing signals orby an oscillator. The wave forms produced by these circuits havedeviations from the exact saw-tooth wave form desired because of thefact that neither the resistance nor the diode at saturation produces aconstant plate current with changes in plate voltage.

It is an object of this invention to provide improved televisionscanning means.

It is another object of this invention to provide means for producing animproved sawtooth wave form adapted for use in a sweep circuit.

t is a further object of this invention to provide improved means formaintaining the current in a circuit substantially constant undervarying impressed voltages.

In carrying these objects into effect, there is preferably employed apentode tube in series with a battery to charge up a condenser orcondensers in a sweep circuit, these being discharged by the periodicdischarge of a 'Ihyratron tube actuated by synchronizing signals.Preferably, the pentode tube is connected so that the current in theoutput circuit of the tube controls the bias on either the control grid,the screen grid, the suppressor grid, or any combination of these three.It has been found that the pentode gives a flatter plate current versusplate voltage characteristic than a triode and for this reason ispreferred. A triode or a tetrode with a feedback connection to a grid orgrids may be used but it has been found that the pentode type of tubewith feedback on one or more of its grids is particularly effective. Ifthe feedback is to be to a single grid, the control grid is preferred.Somewhat better results are obtained by including also the screen gridor suppressor grid, or both, in the feedback connection. By means ofthis arrangement of connections, as the plate voltage decreases, thebias on one or more of the grids is 55 reduced and hence the currenttends to remain constant. With this feedback hook-up, the pentode tubetends to give a very stable output current regardless of voltagechanges. The resultant sweep circuit voltage is thus of the desiredsaw-tooth wave form to produce correct linear scanning.

The invention will be more readily understood from the followingdescription taken in connection with the accompanying drawing forming apart thereof in which:

Fig. 1 shows a sweep circuit for producing sawtooth wave forms;

Figs. 2 and 3 show modifications of the hook-up for the pentode tubeshown in Fig. 1; and

Figs. 4 and 5 are graphic representations included to better explain theoperation of this invention.

Referring more particularly to the drawing, Fig. 1 shows a sweep circuitfor a cathode ray oscillograph tube in which a pentode tube it is used.The circuit comprises a means such as battery II for charging condensersl2 and it through the pentode tube it, and a Thyratron tube M fordischarging these condensers at predetermined periodic intervals, thesedischarges being actuated by synchronizing signals. A suitable source ofsynchronizing signals, represented generally by the box IE, maycomprise, for example, a receiver for receiving these signals from atransmitting station or it may be a local oscil-v lator at the stationwhere the sweep circuits are located. The source 15 is coupled by meansof a transformer l6 and resistances I1 and It to the grid I9 of theThyratron tube I4. The grid is normally negatively biased by the battery20 located between the cathode 2i and the grid IS of the 'Ihyratrontube. In the plate circuit of the Thyratron tube I4 is a resistance 22and one set of sweep plates 23 and 24 of a cathode ray device.

The voltage applied across the sweep plates 23 and 24 is determined bythe charging and discharging of the condensers l2 and i3 and the voltagewave form follows the general form of the full line wave shape 50 shownin Fig. 4. The condensers l2 and I3 may be, if desired, replaced by asingle condenser. In the preferred embodiment of this invention,however, two condensers in series are used with their common terminalconnected to a point of fixed potential in order to produce a balancedsweep circuit. The potentials generated by the sweep circuit maybebalanced with respect to the potential of the accelerating anode 45 ofthe cathode ray tube by using coupilng condensers 43 and couplingresistor 44. For a more complete description of balanced sweep circuits,in general, reference should be made to an application filed February25, 1936 Serial No. 65,606, by Frank Gray.

The elements of the pentode tube Ill and the connections therefor willnow be described. This tube comprises a cathode 25, a heater element 26for the cathode, a plate or anode member 21, a control grid 28, a screengrid 29 and a suppressor grid 36. Current for the heater element 26 issupplied by any suitable means such as the secondary 3| of atransformer, the mid-point of this secondary being connected to thecathode 25. A battery 32 places the screen grid 29 at a. positivepotential with respect to the cathode, preferably at a potential betweenthat of the cathode 25 and the plate 21. The suppressor grid 30 isconnected to the cathode 25 and this connection may be, as desired, madeeither in the tube envelope or externally. Bias is supplied to thecontrol grid 28 of the tube ID by means of the resistance 33 which maybe, if desired, variable. This biasing resistance 33 serves a doublepurpose. First, it takes the place of a biasing battery, and second, asit is in the circuit including the battery II, condensers I2 and I3 andthe plate circuit of the device I0, it serves to change the bias on thecontrol grid 28 as the current in the external circuit changes. Ifdesired, a battery may be included in the circuit including the cathodeand the resistance 33 to produce a fixed bias. Thus, it can be said thatthere is a feedback from the plate circuit to the grid circuit of thedevice Ill. Its function will now be considered.

In order to better explain the operation of the device ID, referencewill now be made to Figs. 4 and 5. In Fig. 5, curve 40 represents atypical plate current versus plate voltage characteristic -of a diodetube which has been customarily used in cathode ray sweep circuits ofthe past. It will be noted that while the curve tends to flatten out ashigher voltage values are reached, it. is still not very close to theideal constant current characteristic desired. The ideal characteristicis represented by the dotted line 4|. In the ideal sweep circuit, thecurrent in the condenser charging circuit must remain constant even withchanges in voltage if the rate of charge with respect to time is toremain linear. In order to bring this out more fully, the operation ofthe sweep circuit will now be described.

The battery I I charges the condensers I2 and, I3 in series through thedevice I0. As the potential between points A and B rises as thecondensers are charged, the potential drop across the tube I fallsbecause the battery I I supplying the electromotive force for thisseries circuit tends to produce a substantially constant potential. Thisdrop across the device I0, assuming that it is an ordinary resistance ora diode, would cause a decrease in the current in the circuit whichwould tend to charge the condensers at a slower rate. The dotted curve5| of Fig. 4 illustrates the effect of this decrease in the chargingrate. This departure from linearity in the saw-tooth wave form willcause distortion of the image on the fluorescent screen of the cathoderay tube. The use of the pentode in place of the diodewill materiallyimprove the linearity of the wave form as pentodes have an inherentlyflatter plate current versus plate voltage characteristic. In Fig. 5 thecurve 42 represents the characteristic of an ordinary pentode tube(without feedback). If the pentode is connected as shown in Fig. 1, thecharacteristic tends to approach the ideal charproach the idealsaw-tooth form 50, shown in full lines in Fig. 4.

When the charge across the condensers has been allowed to build upduring the desired time interval, the condensers are discharged, thisdischarge producing the fly-back of the cathode ray beam. Let it beassumed, for example, that the sweep circuit of Fig. 1 is to be used toproduce horizontal deflection of the beam. At the end of each line ofthe picture, a synchronizing signal will cause the Thyratron tube I4 tobe discharged and the electron beam will fly back to place itself inposition to scan the next line. The condensers will then begin to chargeagain and the entire cycle will be repeated over and over again at afrequency depending upon the frequency of the synchronizing signalsgenerated by the source I5. Once every cycle, therefore, the source I5supplies a signal which overcomes the negative bias of the Thyratrontube I4 to cause a discharge to be initiated which in turn causes thecondensers I2 and I3 to be discharged through the resistance 22 and thedischarge path of the Thyratron I4. When the potential between plate andcathode of the Thyratron tube drops to a low value due to the dischargeof the condensers, the discharge current stops and the grid I9 regainscontrol of the Thyratron and thus is in a position to initiate the nextdischarge at the proper time.

The change in potential between the points A and B may be impressedbetween the deflecting plates 23 and 24 of a cathode ray tube by anydesirable means such as, for example, by coupling condensers 43 andresistance 44, the mid-point of which resistance is preferably connectedto the accelerating anode 45 of the cathode ray tube.

To produce vertical deflection, a circuit similar to that of Fig. 1would be used. The frequency of the wave form produced by this circuitwould, of course, be much smaller than that of the circuit producinghorizontal deflection, as in television the frequency of the deflectionsproduced by this circuit bears to that of the circuit producinghorizontal deflection some definite ratio, and the circuit constantswould therefore be different. Aside from these differences, however, thedescription and operation described above in connection with thehorizontal sweep circuit apply equally as well to a circuit forproducing vertical deflection. The above description applies to the casewhere horizontal scanning is used. It is obvious, however, that thesweep circuits may be reversed and vertical scanning used instead.

Figs. 2 and 3 show modifications of the method of connecting theelements of the pentode tube I3, shown in Fig. 1. Either of the hook-upsshown in Figs. 2 and 3 may be substituted for the portion of Fig. 1below the line X--X. In both Fig. 2 and Fig. 3 the pentode tube I0contains exactly the same elements as the tube used in Fig. 1, but theexternal connections are slightly modified.

In Fig. 2, the screen grid instead of being at a fixed potential withrespect to the cathode 25 (as in Fig. 1) is arranged by means of abattery 46 connectedbetween the screen grid 29 and the biasingresistance 33 to give feedback on the screen grid aswell as on thecontrol grid 28.

The efiect of having feedback on both the con--- trol grid 28 and thescreen grid 29 is cumulative and tends to produce a characteristic veryclose,

to the ideal characteristic 4!.

Fig. 3 differs from Fig. 2 in that feedback is applied to the suppressorgrid 30 as=well as to the control grid 28 and the screen grid 29; Thisfeedback is obtained by connecting suppressorchanges the bias on thethree grids 28, 29 and 38' with respect to the cathode 25 and has theefiect of keeping the current through the tube substantially constant.

It will be obvious that other combinations may be used as well as thoseabove described and that this invention in certain aspects may becarried out with any electron discharge tube arrangement.

Various other modifications may be made without departing from thespirit of the invention, the scope of the invention being defined by theappended claims.

What is claimed is:

i. A linear time axis generator comprising two condensers connected inseries with their common terminal placed at a fixed potential, meanscomprising a pentode electron discharge device and a source of directcurrent for charging said condensers at a substantially uniform rate,and means for periodically discharging said condensers simultaneously.

2. A sweep circuit for a cathode ray tube comprising two condensersconnected in series with their common terminal placed at a fixedpotential, an electron discharge device, having at least one grid,connected in circuit with said condensers, and means for controlling thebias of said grid in accordance with the current through said device.

3. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electron discharge device having a cathode, ananode, a control grid, a screen grid, and a suppressor grid, twocondensers connected in a series circuit with said cathode and anodewith the common terminal of said condensers placed at a fixed potential,means for producing a flow of current between said cathode and anode,and means including apparatus responsive to said flow of current andwhich controls the biasing potential between said cathode and saidsuppressor grid for controlling said flow of current in such a mannerthat it tends to remain substantially constant in intensity.

4. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electron discharge device having a cathode, ananode, a control grid, a screen grid, and a suppressor grid, twocondensers connected in a series circuit with said cathode and anodewith the common terminal of said condensers placed at a fixed potential,means for producing a flow of current between said cathode and anode,and means including apparatus responsive to said fiow of current andwhich controls the biasing potential between said cathode and at leasttwo of said grids for controlling said flow of current in such a mannerthat it tends to remain substantiallyconstant in intensity.

5. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electronv discharge device having a cathode,

an anode, a control grid, and a screen grid, two condensers connected inaseries circuit with said cathode and anode with the common terminal ofsaid condensers placed at a fixed potential, means for producing a flowof current between said cathode and anode, and meansincluding apparatusresponsive to said flow of current and which controls the biasingpotential between said cathode and said control and screen grids forcontrolling said flow of current in such a manner that it tends toremain substantially constant in intensity.

6. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electron discharge device having a cathode, ananode, acontrol grid, a screen grid, and a suppressor grid, twocondensers connected in a series circuit with said cathode and anodewith a common terminal of said condensers placed at a fixed potential,means for producing a flow of current between said cathode and anode,and means including apparatus responsive to said fiow of current andwhich controls the biasing potential between said cathode and saidcontrol and suppressor grids for controlling said flow of current insuch a manner that it tends to remain substan tially constant inintensity.

7. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electron discharge device having a cathode, ananode, a control grid, a screen grid, and a suppressor grid, twocondensers connected in a series circuit with said cathode and anodewith the common terminal of said condensers placed at a fixed potential,means for producing a flow of current between said cathode and anode,and means including apparatus responsive to said flow of current andwhich controls the biasing potential between said cathode and saidsuppressor and screen grids for controlling said flow of current in sucha manner that it tends to remain substantially constant in intensity.

8. An electron discharge device arrangement for use in cathode ray sweepcircuits comprising an electron discharge device having a cathode, ananode, a control grid, a screen grid, anda suppressor grid, twocondensers connected in a series circuit with said cathode and anodewith the common terminal of said condensers placed at a fixed potential,means for producing a flow of current between said cathode and anode,and means including apparatus responsive to said flow of current andwhich controls the biasing potential between said cathode and all threeof said grids for controlling said flow of current in such a manner thatit tends to remain substantially constant in intensity.

9. A linear time axis generator comprising two condensers connected inseries with the common terminal of said condensers placed at a fixedpotential, means including a constant current device and a directcurrent source in series with said condensers for charging saidcondensers, said constant current device comprising an electron saidgrids to control the bias on at least one of charge device to therebydischarge said consaid grids to thereby keep the flow of current throughsaid device substantially constant, a gasfilled electron dischargedevice having an input and an output circuit, said output circuitinclud- Ling said condensers, and signal responsive means forperiodically starting a discharge in said gasfilled discharge device tothereby discharge said condensers.

18. A linear time axis generator comprising two condensers connected inseries with the common terminal of said condensers placed at a fixedpotential, means including a constant current device and a directcurrent source in series with said condensers for charging saidcondensers, said constant current device comprising an electrondischarge device having a cathode, an anode, a control grid, a screengrid, and a suppressor grid, means including said direct current sourcefor maintaining a flow of current in saidv discharge device, meansincluding a resistance in circuit between said cathode and said controlgrid to control the bias on said grid to thereby keep the flow ofcurrent through said device substantially constant, a gas-filledelectron discharge device having an input and an output circuit, saidoutput circuit including said condensers, and sig nal responsive meansfor periodically starting a discharge in said gas-filled dischargedevice to thereby discharge said condensers.

11. A linear time axis generator comprising two condensers connected inseries with the common terminal of said condensers placed at a fixedpotential, means including a constant current device and a directcurrent source in series with said condensers for charging saidcondensers, said constant current device comprising an electron ischargedevice having a cathode, an anode, a control grid, a screen grid, and asuppressor grid, means for fixing the biasing potentials of said controland suppressor grids with respect to said cathode, means including saiddirect current source for maintaining a flow of current in saiddischarge device, means including a resistance in circuit between saidcathode and said control grid to control the bias on said grid tothereby keep the flow of current through said device substantiallyconstant, a gas-filled electron dis charge device having an input and anoutput circuit, said output circuit including said condensers, andsignal responsive means for periodically starting a discharge in saidgas-filled disdensers.

12. A linear time axis generator comprising two condensers connected inseries with the common terminal of said condensers placed at a fixedpotential, means including a constant current device and a directcurrent source in series with said condensers for charging saidcondensers,

said constant current device comprising an electron discharge devicehaving a cathode, an anode, a control grid, a screen grid, and asuppressor grid, means for fixing the biasing potential of saidsuppressor grid with respect to said cathode, means for fixing thebiasing potential of said screen grid with respect to said control grid,means including said direct current source for maintaining a flow ofcurrent in said discharge device, means including a resistance incircuit between said cathode and said control grid to control the biason said grid to thereby keep the flow of current through said devicesubstantially constant, a gas-filled electron discharge device having aninput and output circuit, said output circuit including said condensers,and signal responsive means for periodically starting a dis charge insaid gas-filled discharge device to thereby discharge said condensers.

13. A linear time axis generator comprising two condensers connected inseries with the common terminal of said condensers placed at a fixedpotential, means including a constant current device and a directcurrent source in series with said condensers for charging saidcondensers, said constant current device comprising an electrondischarge device having a cathode, an anode, a control grid, a screengrid, and a suppressor grid, means for fixing the biasing potentials ofsaid screen and suppressor grids with respect to that of said controlgrid, means including said direct current source for maintaining a flowof current in said discharge device, means including a resistance incircuit between said cathode and said control grid to control the biason said grid to thereby keep the flow of current through said devicesubstantially constant, a gas-filled electron discharge device having aninput and an output circuit, said output circuit including saidcondensers, and signal responsive means for periodically starting adischarge in said gas-filled discharge device to thereby discharge saidcondensers.

FRANK R. NORTON.

